Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric Motor Question: Round 2

  1. May 22, 2014 #1
    So I recently asked a question regarding the power consumption of an electric motor. The load that the motor moved equated to approximately .02 horse power. Because acceleration was very slow, the inertia of the motor rod/components should not be of too much concern. However, I understand that they're not negligible.

    My next question arises from the recent use of simulation software that I used to confirm this. The software gave a similar power requirement to move the load. Roughly .02 horse power again.

    This software also served to recommend motor and drive models. What I found surprising was the recommendation of motors and drives rated up to 4 KWs. Way beyond the power needed to move the load. I thought this was odd, but now am thinking I figured out why. So I'm wondering if anyone could confirm or correct the reasoning I'm about to provide.

    Although the power needed to move the load is relatively low, that doesn't account for the individual factors that make up power: torque and speed. I simulated my application with a constant load which moved very slowly, but also needed to provide a somewhat large amount of torque. Between the high torque and slow speed, the slow speed causes the power the be relatively low. Regardless, there is still a high amount of torque needed.

    Because current is proportional to torque in electric motors, high torque will require high current. Because of this, is it correct to assume that this may require the use of a higher power motor/drive due to the fact that it needs to handle a high current? Although power is only required to be .03 horse power, the amount of current to produce that can not occur at typical motors rated at that low of power typically?

    Hopefully you understand what I'm asking.

    Thank you
  2. jcsd
  3. May 22, 2014 #2
    What kind of motor is it?
  4. May 22, 2014 #3
    12 RPM is very slow. Maybe you should think about some kind of gearing. Your problem might be fairly weird design requirements that are outside the bounds of the software. Stalling isn't good on electric motors.
    Last edited: May 22, 2014
  5. May 23, 2014 #4


    User Avatar
    Gold Member

    Not sure where 12 RPM came from (maybe the previous post?) but small dc motors run most efficiently at about half of their no load RPM (very roughly). Gearing is in order here.

    At low speeds there is little back emf, so the full voltage is applied across the motor resistance, meaning very high current.
    Here is a sample curve. http://www.micromo.com/motor-calculations.aspx

    As for why 4KW motors are reccommended, it could be because that's what they sell or what MrSParkle said. Hard to say without playing with the tool.

    Try assuming a large gear ratio and see what it says.
  6. May 23, 2014 #5

    120V single phase servo motor
  7. May 23, 2014 #6

    That's what I'm beginning to realize.
  8. May 23, 2014 #7
    Yeah the 12 rpm comes from the previous post.

    So if I were to increase the speed directly out of the motor, I would get the same power output but with higher speed and lower torque. This would then be geared to the produce the necessary speed and torque at the load side.

    When I ran this option in the simulation software, i had many more options pop up and the motors were closer to the half horse power range.
  9. May 23, 2014 #8
    0.02 HP vs 0.5 is still a poor ratio -- is this a test case or something you really want to build? A small stepper motor can run like this - you did not list the torque - I am getting about 9 Ft#... 100 in#
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook